[PATCH v2 3/5] KVM: x86/tdp_mmu: WARN if PFN changes for spurious faults

[Yan Zhao <yan.y.zhao@intel.com>]

Add a WARN() to assert that KVM does _not_ change the PFN of a
shadow-present SPTE during spurious fault handling.

KVM should _never_ change the PFN of a shadow-present SPTE and TDP MMU
already BUG()s on this. However, spurious faults just return early before
the existing BUG() could be hit.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 6365eb6c1390..d219ecd4ac5b 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1139,8 +1139,10 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
 
 	if (is_shadow_present_pte(iter->old_spte) &&
 	    (fault->prefetch || is_access_allowed(fault, iter->old_spte)) &&
-	    is_last_spte(iter->old_spte, iter->level))
+	    is_last_spte(iter->old_spte, iter->level)) {
+		WARN_ON_ONCE(fault->pfn != spte_to_pfn(iter->old_spte));
 		return RET_PF_SPURIOUS;
+	}
 
 	if (unlikely(!fault->slot))
 		new_spte = make_mmio_spte(vcpu, iter->gfn, ACC_ALL);
-- 
2.43.2